A susceptor having the most basic structure has a three-layer structure including a first and a second transparent quartz part and an opaque quartz part sandwiched therebetween. For example, the opaque quartz part is made of "foamed quartz". In addition, the opacity of the opaque quartz part to flash light is determined to fall within an appropriate range based on the material or thickness of the opaque quartz part, taking into consideration the composition or thickness of a thin film formed on the substrate and various conditions concerning the energy of the irradiation light during flash light irradiation or the like. The stack structure may be composed of a stack of a plurality of opaque quartz layers having different opacities.